1. Field of the Invention
The present invention relates to a technology for reducing noise generated in an image, and in particular, to a technology for reducing noise generated in each row of an image.
2. Description of the Related Art
A known radiographic apparatus uses a flat panel detector made of amorphous silicon or polysilicon deposited or formed on a glass substrate. Original image data acquired by the flat panel detector includes noise components generated in the individual rows of an image, in addition to a pure signal component converted from incident X-rays. The flat panel detector reads the image data as an image signal by turning on and off semiconductor switches present in the same pixel row. In this case, temporal changes in gate signals for the turning on/off are considered to be one of the causes of noise generated in the individual rows of an image.
A method for reducing linear noise generated in the individual rows of an image (referred to as “horizontal noise” in Japanese Patent Laid-Open No. 2003-204955 (herein after referred to as JP-A-2003-204955)) is disclosed in JP-A-2003-204955 (FIG. 3 and so on).
The noise reducing technology described in JP-A-2003-204955 extracts noise components generated in the individual rows of an image by executing high-pass filtering in the column direction of an image and low-pass filtering in the row direction and subtracts the extracted noise components from the original image data.
However, the method described in JP-A-2003-204955 includes not only “horizontal noise” but also signal components that constitute an object. In particular, a region on the image of the object region where the values of pixels sharply changes contains the signal component of the object. Therefore, the “horizontal noise” disclosed in JP-A-2003-204955 is influenced by the acquired object image. This may cause the S/N ratio of the image signals to decrease which may be undesired.
To increase the reading speed, in general, a method for dividing a flat panel detector into multiple regions and reading image signals with different amplifiers for the individual divided regions is adopted. However, this may result in undesired noise quantity differences from one amplifier to another because of the differences in characteristic among the amplifiers.